1. Field of the Invention
The present invention relates to an implantable tubular prosthesis having a textile substrate with a fluid-tight microporous lining.
2. Description of the Prior Art
Tubular prostheses are commonly used as vascular grafts to replace damaged or diseased veins and arteries. To maximize the effectiveness of any prosthesis, it is desirable that the prosthesis have characteristics which closely resemble that of the natural body lumen which it is replacing.
Presently, conventional tubular prostheses and, more specifically, vascular grafts are formed by either weaving, knitting or braiding synthetic fibers into a tubular structure or using a polymer such as polytetrafluoroethylene to create a tubular structure for use as a prosthesis. Tubular textile structures have the advantage of being naturally porous, which allows desired tissue ingrowth and assimilation into the body. Porosity must be balanced to allow for ingrowth of surrounding tissue, yet minimize leakage during the initial implantation. Attempts to control porosity and provide a sufficient fluid barrier have focused on tighter stitch construction such as knitted or woven double-velours and biodegradable natural coatings such as collagen or gelatin. While these grafts sought to overcome the difficulties in achieving the porosity/fluid-tight balance, they failed to adequately address the natural tendency of tubular structures to kink or collapse when the graft is twisted or bent during or subsequent to implantation. Thus, the prior art solutions to the porosity/fluid-tight balance left unanswered the problems of kinking and overall handling.
One conventional solution to the kinking and collapsing problems has focused on the reinforcement of the prosthesis walls using reinforcing fibers, rings, or bands circumferentially placed on the tubular structure. Additional reinforcement of this kind, however, has the disadvantage of reducing the radial and/or longitudinal compliance of the graft due to the increased stiffness of the reinforcing member. A reduction in compliance reduces the area through which blood can flow, thereby compromising the ability of the prosthesis to adjust to body conditions and perform naturally. Additionally, reinforcing members are generally made from solid structural materials which cannot be penetrated by cellular ingrowth from surrounding tissue and may even cause the erosion of the surrounding tissue during contraction.
Another method of increasing the kink and crush resistance of textile grafts is to crimp the graft, i.e., longitudinally compress the tubular structure. Crimping is generally described in U.S. Pat. No. 3,142,067. While crimping serves to add a dimension of kink and crush resistance to the graft, the intraluminal surface formed by crimping includes peaks and valleys which create hemodynamic turbulence within the graft as blood passes therethrough. This turbulence affects the rate of flow and the peaks and valleys formed on the intraluminal surface contribute to excessive thrombus formation and deposition of plaque.
Another disadvantage of presently available tubular textile prostheses, in particular woven and braided grafts, is that sutures tend to pull out or tear the fabric thereby making it difficult to attach the prosthesis to the existing body lumen and to prevent leakage at this junction. Furthermore, textile tubular prostheses formed from a synthetic yarn tend to have ends of the tube which easily ravel. Once the ends ravel or fray, suturing to the existing body lumen becomes extremely difficult.
Microporous tubing formed by stretching polytetrafluoroethylene (PTFE) has also been used as implantable prostheses and especially as vascular grafts. PTFE porous tubes are considered by some to be superior in certain respects to conventional prostheses made of knitted or woven fabrics. The stretched or expanded PTFE tube has a microfibrous structure defined by the presence of nodes inter-connected by fibrils. While PTFE grafts have the advantage of being generally fluid-tight without the use of pre-clotting or specialized coatings, these grafts have limitations in their tear and tensile strength and compliance properties. PTFE grafts often require wrapping with a reinforcing support film to improve undesirable dilation. Reinforcement materials tend to impede the ingrowth of tissue necessary for rapid healing. In addition, PTFE grafts tend to be noncompliant as compared to textile grafts and natural vessels, thereby lacking many of the mechanical properties advantageous to textile grafts.
From the previous discussion it is apparent that both conventional textile prostheses and PTFE prostheses have respective benefits and disadvantages, but neither offers properties which solve all of the aforementioned problems.
Accordingly, it would be advantageous to provide a new and improved implantable tubular prosthesis which combines the best attributes and properties of each of the conventional grafts. More specifically, it would be particularly desirable to form a prosthesis which has the following characteristics: an outer surface porosity which encourages tissue ingrowth into the prosthesis; ravel and fray resistance for better suture retention and tailoring; longitudinal compliance for ease of implantation, sizing and natural vessel simulation; and a fluid-tight lumen without the need for pretreating, coating or pre-clotting.